The invention relates to a control process for increasing the traction in the case of a vehicle, while maintaining the same vehicle stability.
Wheel slip control systems or slip control systems for motor vehicles are sufficiently known. In the case of conventional slip control systems, a compromise is usually accepted between a demanded driving stability and a good traction of a vehicle. In particular, the slip at the driven wheels is usually kept within fixed slip limits. As a result of the accepted compromise between propulsion and driving stability, only insufficient propulsion can be achieved under certain road conditions, for example, on gravel, in sludge or in deep snow.
It is an object of the invention to further develop a control process of the above-mentioned type by which good traction can be achieved while the vehicle stability remains the same even on routes having a loose base.
This object is achieved by a control process for increasing the traction in the case of a vehicle while the vehicle stability remains the same, a wheel slip being determined at the powered wheels and being compared with at least one slip threshold value. The driving power is reduced when the wheel slip exceeds the slip threshold value. At least one slip integral is formed from the speed differentials between the powered and the non-powered wheels, and the at least one slip threshold value is continuously determined as a function of the at least one slip integral.
Accordingly, a wheel slip threshold value is determined and set according to the situation. In this case, at least one slip integral is formed by means of the wheel speeds at the powered and at the non-powered axle and the vehicle reference speed respectively and conclusions are drawn therefrom with respect to the road conditions. As a function of the road conditions (for example, deep snow, routes with a low coefficient of friction, routes with a high coefficient of friction, wet road), the optimal slip will fluctuate considerably. According to the slip integral, the threshold value for the wheel slip is changed and is thus adapted to the driving conditions. Thus, the actual slip value can be optimally increased or decreased. According to the invention, at least one slip threshold value can be adapted. However, several slip threshold values can also be changed correspondingly.
A low wheel slip threshold value is preferably defined in a standard program. Within the scope of the slip integration, the absolute slip as well as the time-related driving torque course are assessed. Furthermore, by way of an existing sensor arrangement of a drive control system used in the vehicle, preferably at least one of the values is determined from the yaw velocity, the steering angle dynamics and the lateral acceleration and is taken into account during the control process.
Thus, for example, starting from a threshold amount of the vehicle""s lateral dynamics, which is to be derived, for example, from the lateral acceleration, a standard program can be selected. This standard program does without a particularly good traction and has the vehicle stability as a priority. In this case, the change-over preferably does not take place discontinuously. On the contrary, the wheel slip threshold value is changed continuously (slip threshold expansion).
A consideration of other driving-dynamic quantities (such as the steering angle activity, the yaw rate course, etc.) may be superimposed on the evaluation of the pure lateral dynamics, which may have the result that, also when the lateral acceleration is low, a standard program with a minimal slip threshold value is used.
According to an advantageous embodiment of the invention, an increase of the wheel slip threshold value takes place to a certain speed of, for example, 50 km/h to 80 km/h. Above this speed, the threshold value can preferably be returned to a standard threshold value. The change of the threshold value can take place in a linear or progressive manner.
On the whole, a process is provided in which, especially on routes with a loose base, good traction can be achieved. This function can be optionally offered to the driver and, when selected, is preferably indicated visually or acoustically. In particular, as a result of the dynamic change-over to standard programs or small slip values, a sufficient stability can be ensured. The information concerning the stability performance is obtained from the values for the steering angle dynamics, the yaw velocity or the lateral acceleration. Naturally, other quantities can also be used which describe the vehicle stability.
In the following, an embodiment of the present invention is indicated which is explained in detail with respect to the attached drawings.